Through present date, a broad range of rheological instrumentation exists for applications in food products, paints and coatings, polymers and composites, asphalt, pharmaceuticals, electronic materials, etc. For specific purposes, special viscometers such as an oscillating capillary viscometer, automated rolling ball viscometer and a high shear capillary viscometer are now being used. However, there are problems with using rheological instrumentation for soft type materials.
Soft materials, which include polymer solutions, surfactant solutions, and biological materials, among others, are characterized by complex structures with multiple characteristic time and length scales; thus their response to external strains has a nontrivial time dependence. One of the most important descriptors of these properties is the complex shear modulus G(t), which is typically measured in the frequency domain. The real part G′(ω) in the frequency domain describes the elastic storage property of the system, while the imaginary part G″(ω) is a measure of the viscous loss behavior. The ability to measure locally the mechanical response of a material to an applied shear strain has a variety of potential applications, especially in biology, where the mechanical properties of the cells and intracellular matter are of utmost importance.
Recently a number of techniques have been developed or suggested for probing the rheological properties of complex materials at a microscopic scale, an area that has come to be known as microrheology. F. C. MacKintosh and C. F. Schmidt, Curr. Opin. In Coll. Interface Sci. 4, 300. Most of the microrheological techniques rely on applying a strain to the fluid through embedded “probe” particles, and the strains result either from imposed force on the probe. Application of an external force, such as a magnetic field, to the particles may produce a nonlinear response of the fluid since the resulting strain could be substantial, whereas strains resulting from thermally excited probes are quite low and ensure linearity of response in most of the cases. The probe motion is quantified by its mean-squared displacement, which can be obtained by particle tracking, single-particlemicroscopy, or dynamic light scattering (e.g., diffusing wave spectroscopy, when multiple scattering is dominant. See F. Amblard, A. C. Maggs, B. Yurke, A. N. Pargellis, and S. Leibler, Phys. Rev. Lett. 77, 4470(1996); T. G. Mason, K. Ganesan, J. H. Van Zanten, D. Wirtz, and S. C. Kuo, Phys. Rev. Lett. 79, 3282(1997); F. Gittes, B. Schnurr, P. D. Olmsted, F. C. MacKintosh, and C. F. Schmidt, Phys. Rev. Lett. 79, 3286(1997); and T. G. Mason and D. A. Weitz, Phys. Rev. Lett. 74, 1250(1995). It is important to note that recording individual particle trajectories typically requires a smaller sample volume than in the case of diffusing wave spectroscopy [DWS], in which the measurements are performed over an ensemble of particles.
Dynamic light scattering (DLS) techniques have the advantage of providing an inherent average over the particle ensemble, while single-particle techniques require successive measurements in order to obtain a reliable average. It is worth noting that atomic force microscopy (AFM) can also be used in this context. H. Ma, J. Jimenez, and R. Rajagopalan, Langmuir, 16, 2254(2000). Here, one monitors the thermal fluctuations of the tip of the AFM cantilever instead of following a probe particle. While an AFM allows one to examine microrheology at interfaces as do particle-tracking techniques or the dynamic light scattering technique this invention is that the analysis of the AFM data is not straightforward for a number of reasons. Most notable among these are the complicated geometry of the probe and the AFM tip and the influence of the intrusive, mechanical behavior of the AFM cantilever itself and the resulting, often ill-characterized, fluid response. H. Ma, J. Jimenez, and R. Rajagopalan, Langmuir, 16, 2254(2000).
Microrheological measurements have been reported in the last few years for a number of materials, e.g., colloidal dispersions, polymer solutions, and biological cells and materials.
For example, numerous experiments on action networks have shown nontrivial high-frequency dependence of the shear modulus and models have been proposed to explain such behavior. See F. Gittes and F. C. MacKintosh, Phys. Rev. E. 58, R1241(1998); D. C. Morse, Phys. Rev. E. 58, R1237(1998); and J. Xu, A. Palmer, and D. Wirtz, Macromolecules 31, 6486(1998).
It is commonly agreed that most viscoelastic fluids have shear moduli that follow a power-law dependence in the high-frequency region, but the actual value of the exponent appears to vary depending on the materials and, sometimes, the experimental technique used. More recently, it has been shown that cross correlating the thermal motion of pairs of embedded particles offers a more precise and a different way of examining the microrheology of soft condensed matter, suggesting that measurements using previous microrheological techniques may need to be revised or reexamined. See for example, J. C. Crocker, and M. T. Valentine, E. R. Weeks, T. Gisler, P. D. Kaplan, A. G. Yodh, and D. A. Weitz, Phys. Rev. Lett. 85, 888(2000); A. J. Levine and T. C. Lubensky, Phys. Rev. Lett. 85, 1774(2000); and G. Popescu and A. Dogariu, Opt. Lett. 26, 551(2001). Therefore, it is of great interest to find complementary investigation methods for high-frequency viscoelastic behavior of complex fluids at microscopic scales.
For measuring rheological properties, various patents have also been proposed over the years. See for example, U.S. patents: U.S. Pat. No. 5,155,549 to Dhadwal; U.S. Pat. No. 5,457,526 to Kosaka; U.S. Pat. No. 5,459,570 to Swanson et al.; U.S. Pat. No. 5,751,424 to Bostater, Jr.; U.S. Pat. No. 5,986,277 to Bourque et al.; U.S. Pat. No. 5,991,697 to Nelson et al.; U.S. Pat. No. 6,015,969 to Nathel et al.; U.S. Pat. No. 6,175,669 to Colston et al. and U.S. Pat. No. 6,201,608 to Mandella et al. However, the entire prior art systems have various deficiencies.
All the conventional methods generally rely on sensing the effect of a macroscopically-induced mechanical stress. The frequency ranges of these prior art systems are generally limited at several kHz. The prior art systems generally require relatively large volumes of material (at least a few micro-liters) to be available for analysis. For rheological analysis of biological media, the available prior art instrumentation requires sample manipulation and preparation.
To the inventors' knowledge, there are no applicable real in-situ methods systems available to overcome the problems described above.